As part of a class about designing for the plastic injection molding process, I designed, cut molds and shot some napkin rings called the Impaled Napkin Rings series.  Process below.

I first 3D printed some prototypes to see how they worked.  I learned that living hinges do not work with our printer material.  I cut the molds using a HAAS Office Mill OM1.  I had to recut one side as the living hinge at the neck was so small on the first iteration that when the molds separated after shooting they would decapitate the napkin ring (if that makes sense).  I widened the neck and they shot perfectly.